Metal cords for rubber reinforcement and tires using the same

ABSTRACT

A metal cord usable for the reinforcement of rubber articles is comprised of a helically formed single wire filament having a twisting angle of 75-84.5 degrees or a bundle of two twisted metal filaments each having a twisting angle of 75-84.5 degrees. Such metal cords are preferably used as a rubber reinforcement in each part of a pneumatic tire.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a metal cord having a novel twisting structureused as a rubber reinforcing material in tires for a vehicle, a conveyorbelt and the like as well as a pneumatic tire using such metal cords asa reinforcing material.

2. Disclosure of the Related Art

In the art, steel cords of so-called 1×4 or 1×5 twisting structurecomprised of 4-5 filaments are widely used as a rubber reinforcement fora tire or the like. Recently, the use of steel cords having 1×2 twistingstructure has started for attaining the weight reduction, low fuelconsumption and low cost of the tires for vehicles, and also the use ofsingle wire cord comprised of only wire filament is proposed.

However, when the cord of the twisting structure simplified by merelyreducing the number of filaments such as 1×2 twisting structure, singlewire cord or the like is used in the belt of the tire, if the excessiveforce is given in cornering, a so-called buckling deformation is causedin the ground contacting portion outside the belt. Hence the cordbreakage is caused due to the deformation input. Therefore, there isadopted a procedure of adding a new reinforcing material to the edgeportion of the belt in the building of the tire.

Since the use of such a new reinforcing material is contrary to thepurpose for attaining the weight reduction, low fuel consumption and thelike of the tire, it largely decreases the merit of using steel cord of1×2 twisting structure or single wire cord as a rubber reinforcement.

The steel cord of 1×2 twisting structure is proposed in Japanese Patentlaid open No. 62-117893 and No. 62-234921.

However, such steel cords are very poor in the breaking property becausethe angle of steel filament itself too rises with respect to thelongitudinal, direction of the cord, consequently, they are not used inthe belt portion of the tire as they are.

Also, Japanese Patent laid open No. 2-229286 proposes steel cords of 1×2twisting structure, in which forming ratios of two filaments are changedto periodically and alternately create portions of contacting thefilaments with each other and portions of causing no filament contact(portion surrounded by rubber) in the longitudinal direction of thecord.

In the later article, it is described that when the steel cord formed bychanging the forming ratios of the filaments is applied to the belt,portion of the tire, it prevents penetration of water invaded from cutfailure on the tread surface into the steel cord of the belt layer inthe longitudinal direction of the cord. Thus, the cord breakage due tothe occurrence of rust can be prevented.

In the steel cord of 1×2 twisting structure, however, it is notnecessarily required to alternately create the filament contactingportions and the non-contact portions as described the above articlebecause the steel cord of 1×2 twisting structure does not produce cordbreakage and the like due to the occurrence of rust through penetrationof water. Moreover, it is substantially impossible to embed such steelcords in rubber for the tire while alternately holding the contactingportions and the noncontact portions in the cord. Even if these cordsare embedded in the rubber at such a state that the contacting portionsand the noncontact portions are alternately existent in the longitudinaldirection of the cord, the angle of the filament itself with respect tothe longitudinal direction of the cord at the contacting portion is toolarge, so that the cord breakage may be caused.

On the other hand, the single wire cord is described in Japanese Patentlaid open No. 57-198101 and No. 1-229704, which is a simple straightwire cord, so that the resistance to cord breakage is very poor unless areinforcing member such as a layer material or the like is used.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to improve the resistanceto cord breakage in the metal cord of 1×2 twisting structure or singlewire without using the other reinforcing member for attaining the weightreduction, low fuel consumption and low cost of the rubber articles.

The inventors have made various studies in order to solve the problemsof the conventional metal cord of 1×2 twisting structure or single wireand found that the resistance to cord breakage is improved by adoptingthe following structure.

That is, a first aspect of the invention lies in a metal cord formed bytwisting two metal filaments at the same pitch, characterized in that atwisting angle α₁ is represented by an equation (1) of 75°≦α₁ ≦84.5° andthe filaments are contacted with each other at two positions per onetwisting pitch.

In this case, each of these filaments has a filament diameter of0.15-0.8 mm, preferably 0.20-0.6 mm.

A second aspect of the invention lies in a metal cord comprised of ahelically formed single wire, in which a twisting angle α_(M) isrepresented by an equation (2) of 75°≦α_(M) ≦84.5°.

In this case, the single wire has a filament diameter of 0.20-1.2 mm,preferably 0.25-1.0 mm.

According to a third aspect of the invention, there is the provision ofa pneumatic tire comprising a carcass, a belt and a chafer as a tireskeleton member, characterized in that the metal cord defined in theabove first and second aspects of the invention is used in at least oneof these skeleton members.

According to the invention, it has been found that the resistance tocord breakage is improved by using the metal cord defined in the firstand second aspects as a rubber reinforcement, and that when these cordsare applied to the tire, the weight reduction, low fuel consumption andlow cost of the tire can be attained.

The term "twisting angle" used herein means a complement of an angle ofa metal filament with respect to the longitudinal direction of the cord.Considering a case that the cord is true circle, the twisting angle isshown in FIG. 1, which can be considered as an extended cord shown inFIG. 2. In these drawings, numeral 1 is a metal filament, letter P atwisting pitch, letter R a cord diameter, letter d a filament diameter,and letter R₀ a cord radius (R₀ =R/2-d/2).

The twisting angle is shown by a letter α and has the followingrelation:

    α=tan.sup.-1 P/π(R-d)

which is determined by a twisting pitch or forming pitch P, filamentdiameter d and cord diameter R from the following relation:

    tan α=P/2πR.sub.0 =P/π(R-d)

Thus, the twisting angle is theoretically determined by the corddiameter, pitch (twisting pitch in case of 1×2 structure, forming pitchin case of single wire) and filament diameter, and can actually bemeasured from the shape of the filament 1 itself as shown in FIG. 3.

In case that the shape of the cord is ellipsoidal, it is particularlypreferable to actually measure the twisting angle, in which an averagevalue is adopted by measuring at 10 positions in the longitudinaldirection of the cord.

According to the invention, the section of the cord develops the desiredeffect even in the circular shape and the ellipsoidal shape. When theellipsoidal cords are embedded in the belt layer of the tire, it isdesirable to measure an angle of the filament with respect to thelongitudinal direction of the cord from the tread surface. Moreover, inthe ellipsoidal cord, the ratio of major axis a to minor axis b is notparticularly limited, but the ratio a/b is preferably within a range of1≦a/b≦2 in view of the prevention of cord breakage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein:

FIG. 1 is a schematic view showing a part of a metal filament in thelongitudinal direction of the metal cord;

FIG. 2 is a schematic view showing an extended state of the metalfilament;

FIG. 3 is a schematic view of a part of the metal cord;

FIG. 4 is a schematic view showing a part of the conventional steel cordof 1×2 twisting structure and sections thereof at various positions inthe longitudinal direction;

FIG. 5 is a schematic view showing a part of steel cord of 1×2 twistingstructure according to the first aspect of the invention and sectionsthereof at various positions in the longitudinal direction; and

FIGS. 6 and 7 are schematic views of embodiments showing a part ofsingle wire cord according to the second aspect of the invention andsections at various positions in the longitudinal direction,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the metal cord according to the first aspect of the invention, asshown in FIG. 5, the twisting angle of the filament with respect to thelongitudinal direction of the cord is considerably large as comparedwith that of the conventional metal cord of 1×2 twisting structure shownin FIG. 4. When the metal cords according to the invention are used, forexample, in a belt layer of a tire, if bending input such as bucklingdeformation or the like is applied to the belt layer, these cords act tomitigate strain at the surface of the filament, whereby the cordbreakage can be avoided.

In the metal cord according to the second aspect of the invention, anadequate angle of the filament with respect to the longitudinaldirection of the cord is held as shown in FIG. 6, so that cord breakagecan be prevented. Moreover, when the metal cord according to the secondaspect of the invention is subjected to forming as shown in FIG. 7, theresistance to cord breakage is considerably improved as compared withthe conventional single straight cord.

In FIGS. 4 to 7, numerals 2 and 3 are metal filaments, and sections ofthe cord (A₁, B₁, . . . ) in the longitudinal direction thereof areshown beneath arrow positions (A₀, B₀, . . . ,), respectively.

According to the invention, it has been found that in the metal cord of1×2 twisting structure or single wire, when the angle of the twistingfilament or single wire filament with respect to the longitudinaldirection of the cord is not less than 5.5°, the resistance of cordbreakage is largely improved as mentioned above. However, when the angleis too large, the lowering of the strength of the metal cord itselfbecomes large and the volume of one metal cord occupied in the beltlayer increases to decrease the end count. As a result the angle of thefilament to the longitudinal direction of the cord is favorably not morethan 15° for holding the strength of the belt itself in the tire.

Particularly, when the metal cord is used in the belt layer of the tire,it is preferable that the angle of the filament with respect to thelongitudinal direction of the cord is within a range of 7.5°-10° from aviewpoint of the resistance to cord breakage and the strength of thebelt itself.

The metal cords of 1×2 twisting structure according to the first aspectof the invention are produced by forming each of metal filaments at agiven forming ratio through a preformer before the twisting and thentwisting two of these formed filaments, or by continuously twisting twofilaments with each other in the longitudinal direction and then forminga bundle of these twisted filaments at a given forming ratio through apreformer.

Similarly, the metal cords of single wire filament according to thesecond aspect of the invention are easily produced by forming thefilament at a given forming ratio through the preformer.

Moreover, these metal cords may be shaped into an ellipsoidal cord bysubjecting the cord to flat working through pushing rolls or the like.

The following examples are given in illustration of the invention andare not intended as limitations thereof.

In this example, the metal (steel) cords according to the invention areapplied to a belt of a pneumatic tire for passenger car.

The tire to be tested has a tire size of 175/70 R13 and comprises twosteel belt layers having a cord angle of 68° with respect to anequatorial plane of the tire, in which a first belt layer has a width of140 mm and a second belt layer has a width of 130 mm.

After the test tire was inflated under an internal pressure of 1.7kgf/cm² and run on a general road over a distance of 40,000 km, it wascut to measure the number of broken positions in the steel cord at thebelt layer.

The measured results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________              Comparative                                                                           Comparative           Comparative                                     Example 1                                                                             Example 2                                                                            Example 1                                                                             Example 2                                                                            Example 3                                                                             Example                                                                              Example                __________________________________________________________________________                                                           4                      Twisting structure                                                                      1 × 4 × 0.25                                                              1 × 2 × 0.30                                                             1 × 2 × 0.30                                                              1 × 2 × 0.30                                                             1 × 2 × 0.30                                                              1 × 1 ×                                                                  1 × 1                                                                   × 0.50           of cord                                                                       Twisting angle                                                                          84.7    86.2   82.5    80.0   74.0    82.5   82.5                   Cord pitch                                                                              14      14     14      14     14      8      8                      End count 40.2    45.7   45.7    40.0   25.0    46.5   45.6                   (cords/50 mm)                                                                 Sectional shape   FIG. 4 FIG. 5  FIG. 5 FIG. 5  FIG. 6 FIG. 7                                   (conventional                        (a/b = 1.4)                              type)                                                       Number of broken                                                                        55      80     20      25     52      26     22                     positions in cord                                                             __________________________________________________________________________

As seen from Table 1, the occurrence of steel cord breakage in the steelcords according to the invention (Examples 1-4) is reduced to a half orless of that in the conventional steel cord of 1×2 twisting structure(Comparative Example 1). Even in the steel cords of 1×2 twistingstructure, when the twisting angle is more than 84.5° (ComparativeExample 2) or less than 75° (Comparative Example 3), the breakage of thesteel cord is equal to or more than that of Comparative Example 1. Whenthe twisting angle is restricted according to the invention (Examples1-3), the occurrence of steel cord breakage is considerably reduced, sothat the restriction of the twisting angle according to the inventiondevelops a great effect. Furthermore, the occurrence of cord breakage isconsiderably reduced even in the metal cords of single wire filament(Examples 3 and 4).

As mentioned above, according to the invention, the twisting angle ofthe filament in the metal cord is restricted to a particular anglerange, whereby the resistance to cord breakage can considerably beimproved. Further, when such metal cords are used, for example, in eachconstituting part of the tire, the weight reduction and low fuelconsumption and low cost of the tire can be attained. Therefore, themetal cords according to the invention may be widely used as a cord forrubber articles.

What is claimed is:
 1. A metal cord formed by twisting two metalfilaments at the same pitch, characterized in that a twisting angle α₁is represented by an equation (1) of 75°≦α₁ ≦84.5° and said filamentsare contacted with each other at two positions per one twisting pitch.2. The metal cord according to claim 1, wherein said filament has afilament diameter of 0.15-0.8 mm.
 3. The metal cord according to claim2, wherein said filament diameter is 0.20-0.6 mm.